Best way to fix this mesh?
If you mean less than perfect in term of the exact volume, than this is problematic for numerical reasons already. A treshold is needed at some point to determine which edges can be removed. 

The order in which you use the operations play a major role. As you mentioned, the Decimate Modifier in Planar Mode is probably your best bet for this. Note that dissolving and decimating will be similar in this case, but I would try Decimate first and then Dissolve in the next step.

Also don't use Convert Tris to Quads before using Dissolve or Decimate, it will make the geometry more messy. It should be the last step and probably guided by hand. Selecting some important loops and using sharp/seem edges will help to get cleaner Quads. Check Compare Sharp or Compare Seam.

`Beautify Faces" might help, but might make it worse aswell. Trying can't hurt.

You might also play with the export settings and try a different file format in SolidWorks.